Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to cellular phones, camcorders, notebook computers, PCs and electric cars, the demand for high energy density of batteries as a power source has been increasing. Lithium secondary batteries have been proposed as a battery that can satisfy such a demand, and their researches are being actively made.
However, the lithium secondary batteries may cause safety problems such as ignition and explosion and are difficult to be produced because an organic electrolytic solution is used therein. Particularly, the lithium secondary batteries have recently been used under various conditions and environments as their application range is greatly expanded. As a result, a demand for lithium secondary batteries with a higher capacity is gradually increasing. In order to provide lithium secondary batteries with a higher capacity, the operation ranges of an electrode tend to be expanded, for example, into a high voltage. Such a high voltage is favorable in terms of battery capacity, but may cause more serious safety problems.
Generally, a lithium secondary battery is prepared by carrying out an activation process that initially charges a battery in the state of discharging. In particular, a lithium-containing compound with a layered structure, represented by the following formula (I), has a specific uniform potential in the region of 4.3 to 4.8 V, unlike other cathode materials that have been conventionally known, and should go through an activation process at high voltage conditions above such uniform potential voltage region so as for the compound to exhibit a high capacity through the structural variation thereof. In the activation process, the lithium-containing compound used as a cathode active material is subject to structural variation at a high voltage, from which large amounts of gases may be generated and remained within the battery to deteriorate the transfer of lithium ions and result in Li plating locally. Therefore, gases generated during the activation process should be removed.
Meanwhile, a lithium secondary battery that has a cathode comprising a cathode active material represented by the following formula (I) exhibits a high capacity when being activated at a high voltage, but has poor life time and low rate characteristics owing to its local structure change, as compared with a lithium secondary battery having a cathode comprising a general layered lithium-metal oxide. In order to compensate such poor life time and low rate characteristics, there has been an attempt that a cathode active material represented by the following formula (I) is mixed with a general layered lithium-metal oxide such as LiNi1−x−yCoxMnyO2 (0≤x≤0.5, 0≤y≤0.5) and LiCoO2 in the preparation of an electrode to obtain a cathode and then a cell. However, when a cell comprising such a mixed cathode active material is initially activated under high voltage, the general layered lithium-metal oxide undergoes durability deterioration to result in the rapid life shortening and performance deterioration of the cell during operation. For this reason, such a cell is difficult to be commercialized.Li(LixMy−y′M′y′)O2−zAz  (I)
wherein, x, y, y′, and z satisfy 0<x<0.5, 0.6<y<1.1, 0≤y′<0.2, and 0≤z<0.2,
M is any one selected from the group consisting of Mn, Ni, Co, Fe, Cr, V, Cu, Zn, and Ti,
M′ is any one selected from the group consisting of Al, Mg and B; and
A is any one selected from the group consisting of F, S and N.